Executive Control and the Prefrontal Cortex

Executive control is the ability to guide thought and action in accordance with internal goals. One of the great mysteries of neuroscience concerns how this capacity for coordinated, purposeful behavior arises from neural states. Decades of cognitive and neuroscientific research have focused on the mechanisms by which executive control guides behavior and the brain structures upon which these functions depend, such as the prefrontal cortex (PFC), anterior cingulate cortex, basal ganglia and brainstem neuromodulatory systems. However, the functional contribution of these regions and their interactive role in the coordination of cognitive, social and affective processes remain to be well characterized. A major goal of our research is to understand the prefrontal networks underlying executive control, investigating the functional profile and topography of lateral and orbitofrontal sectors of PFC.

Lateral Prefrontal Cortex

Accumulating neuroscience evidence indicates that lateral PFC exhibits three properties that are central to executive control: (1) the representational coding scheme of this region conveys information about internal behavioral goals and/or prior temporal context; (2) these representations can be actively maintained over time in a highly accessible form (i.e., storage of information via sustained neuronal activity patterns); and (3) the output of this region is an activation signal that biases the flow of ongoing processing in other brain regions, such as those responsible for perception, action selection, memory retrieval and emotional evaluation. Additionally, the neurotransmitter dopamine plays a key modulatory role over lateral PFC function by regulating the way that goal representations are maintained and updated. These findings elucidate the architecture of executive control in lateral PFC and suggest that this region may represent a necessary component of the neural systems for goal-directed behavior.

Our research has investigated this hypothesis by studying human brain lesion patients with focal damage to lateral PFC, examining whether dorsolateral and ventrolateral sectors are necessary for performance on neuropsychological tests of executive control (Delis-Kaplan Executive Function System) and broader tests of general intelligence (Wechsler Adult Intelligence Scale) (Barbey et al., under revision). The results of this study indicate that focal dorsolateral PFC damage is associated with pervasive deficits on tests of executive control (relative to patients with ventrolateral PFC lesions). Selective impairment was observed in the left dorsolateral PFC patient sample on tests of visual-motor coordination (Visual Scanning and Motor Speed), hypothesis testing (Twenty Questions) and cognitive flexibility (Category Switching). This patient group additionally exhibited deficits in psychometric g, particularly for measures of fluid intelligence. These findings, together with emerging functional neuroimaging evidence, demonstrate that left dorsolateral PFC is a central component of the neural systems underlying executive control and general intelligence, and further suggest that this region provides a unified neural architecture for higher cognitive functions.

We have also actively investigated the role of dorsolateral PFC in executive control processes for working memory (Barbey et al., under revision). The observed functional selectivity of this region suggests that it mediates working memory processes that are specifically engaged in the service of executive control – rather than coupled with mechanisms for the active maintenance of information in working memory as Baddeley’s classic model suggests. To investigate the architecture of control processes in working memory, we administered the Wechsler Memory Scale and N-Back Task to patients with focal dorsolateral PFC lesions. Damage to this region was associated with significant deficits in working memory (relative to patients with ventrolateral PFC lesions). Specifically, the left dorsolateral PFC lesion sample demonstrated selective impairment in the ability to manipulate cognitive representations (Digit/Spatial Span Backward, Letter-Number Sequencing), with intact performance on tests of working memory maintenance (Digit/Spatial Span Forward) and monitoring operations (Zero-Back). These results join a growing body of evidence to support the functional selectivity of left dorsolateral PFC in executive control, suggesting that this region represents a central component of the computational machinery for manipulating information in working memory.

Taken together, these findings support an integrative framework for understanding the functional profile of dorsolateral PFC, demonstrating that this region is necessary for executive control functions underlying visual-motor coordination, hypothesis testing, cognitive flexibility and working memory. The diverse control functions mediated by this region suggest that its neurons adaptively code information for the control of behavior in a broad range of contexts, and support new approaches to understanding the evolution and development of PFC function based on the adaptive re-use of existing neural circuits (e.g., employing dorsolateral PFC to support the spectrum of control functions underlying goal-directed behavior). Our current research further examines the functional networks underlying executive control, providing lesion evidence for a distributed fronto-parietal circuit that principally involves dorsolateral PFC and superior parietal cortex (Barbey et al., in press; Koenigs et al., 2009).

Orbitofrontal Cortex

Increasing neuroscience evidence suggests that orbitofrontal cortex provides a nexus for sensory integration, enabling the coordination of cognitive, social and affective representations in executive control. Our research has investigated the necessity of this region for executive control processes in working memory, administering the Wechsler Memory Scale and N-Back Task to patients with focal orbitofrontal cortex lesions (Barbey et al., 2011). The results of this study indicate that damage to medial orbitofrontal cortex is associated with reliable deficits in working memory (relative to patients with lateral orbitofrontal cortex lesions). Selective impairment was observed on neuropsychological tests of the joint maintenance, monitoring and manipulation of representations in working memory (N-Back), with intact performance on pure tests of working memory maintenance (Digit/Spatial Span Forward) and manipulation (Digit/Spatial Span Backward, Letter-Number Sequencing). These findings contribute to an emerging empirical case for the involvement of medial orbitofrontal cortex in the coordination of multiple cognitive operations, suggesting that this region is engaged when the integration of two or more separate cognitive operations is required for goal achievement.